Field of the Invention
The world's energy demand and consumption is rapidly growing. Price increases of energy from fossil resources and environmental concerns make high energy consumption problematic. The International Energy Agency (IEA) has published data on energy consumption trends. While the total primary energy supply (TPES) was doubled from 1973 to 2010 (from 6107 to 12,717 million tons of oil equivalent, MTOE) and crude oil production increased almost 40% (from 2869 to 4011 million tons), the total final energy consumption showed 31% increase (from 2815 to 3691 MTOE). In the last two decades the oil price has increased nearly five times. During the period of 1973-2010, the CO2 emission was also doubled (from 15.637 to 30,326 million tons CO2). These and additional details regarding energy consumption trends can be found, for example, in, International Energy Agency, Key world energy statistics, OECD/IEA (2012), http://www.iea.org/publications/freepublications/publication/kwes.pdf, the entirety of which is incorporated herein by reference. Currently, the energy used for heating or cooling of buildings constitutes a major part of the total global energy (40%) and water (25%) consumption. Buildings are the source of nearly one third of the greenhouse gas emission. These and additional details regarding energy consumption trends can be found, for example, in UNEP, Sustainable buildings & climate initiative, building and climate change, United Nations Environment Program (2009), the entirety of which is incorporated herein by reference. Therefore, buildings provide a substantial potential for reducing emission at relatively low cost.
The European Union's Energy Efficiency Directive (passed on 25 Oct. 2012) recognizes that “ . . . the rate of building renovation needs to be increased, as the existing building stock represents the single biggest potential sector for energy savings. Moreover, buildings are crucial to achieving the Union's objective of reducing greenhouse gas emissions by 80-95% by 2050 compared to 1990.” These and additional details regarding the European Union's Energy Efficiency Directive can be found, for example, in Directive 2012/27/EU of the European Parliament and of the Council of 25 Oct. 2012 on energy efficiency, amending Directives 2009/125/EC and 2010/30/EU and repealing Directives 2004/8/EC and 2006/32/EC, the entirety of which is incorporated herein by reference.
A second problematic aspect is the relative cost of energy. In most of the equatorial regions, where developing countries are predominantly located, the price for electricity compared to the average income is too high to permit 24/7 active air conditioning of most residential buildings.
Therefore, any design improvements to buildings which will noticeably lower indoor temperatures, especially during dry seasons—i.e., during periods of high solar radiation input, will considerably contribute to well-being and efficiency of its occupants. The disclosed invention enables, among other applications, the construction of new buildings and/or modifications to existing buildings, involving various functional elements and methods of operating them, which consume little or no energy, or where at least the costs for the required energy is small e.g. in comparison with costs associated with conventional active air conditioning.
If operating in conjunction with conventional active air conditioning, the energy requirements thereof are generally reduced. Obviously, such methods and functional elements can not only be beneficially applied in equatorial, tropical, and subtropical regions, but also e.g. in areas like the southwest of the Unites States, or central and southern parts of Europe during summer.
Description of the Related Art
Energy budget and temperature inside predominantly enclosed spaces, such as buildings or similar habitats, depends on several physical, climatic, and human variables. These and additional details regarding energy budget and temperature inside predominantly enclosed spaces can be found, for example, in William Chung, Review of building energy-use performance benchmarking methodologies, Appl. Energy (2011), vol. 88, issue 5, pages 1470-1479, the entirety of which is incorporated herein by reference. From a physical perspective this comprises, among other influences, at least the following: The levels of primary electromagnetic irradiance, reflective properties of surfaces, heat capacity and thermal conductivity of the used materials, spatial distribution of said materials, convective heat transfer, thermal radiation (which in turn depends on reflective and thus also emissive properties of surfaces), as well as resulting air flow in combination with externally imposed air flows.
A considerable portion of the energy, which contributes to temperatures inside predominantly enclosed spaces, can be the result of absorbed solar radiation by the building envelope. This effect typically dominates in tropical and subtropical regions, but it can also apply in moderate climate zones, for example, during summer or times of increased relative temperature (e.g., in North America, Central or Southern Europe, and Australia).
The relevant part of the electromagnetic spectrum can conceptually be divided in Ultraviolet (UV), visible (VIS) [≈0.38 μm-0.7 μm], Near Infrared (NIR) [0.7 μm-3.0 μm], Mid Infrared (MIR) [3 μm-50 μm], and Far Infrared (FIR) [50 μm to 1 mm]. The exact definition of the boundaries of NIR, MIR, and FIR are somewhat arbitrary and different values are used by different scientific communities. (For example, sometimes the NIR range is defined smaller and an additional Short Wavelength IR SWIR is defined.) For physical reasons (the corresponding black body temperatures) we will henceforth follow the values given above.
The disclosed invention relates in some embodiments to predominantly enclosed spaces, which are buildings, or other habitats, and which are exposed to electromagnetic radiation in the UV, VIS, NIR, MIR, and FIR range, including solar radiation. Unless explicitly excluded, the term ‘light’ shall henceforth be understood to apply for all of these spectral ranges, not only to the portion of the spectrum visible to humans.
We will subsequently consider reflectance and reflectivity to be synonymous, describing the ratio of reflected over incident electromagnetic power, and consider only this net result of returned vs. incoming radiation, independent of the thickness of any layer or layers (including interference layers, embedded nano- and/or microparticles or cavities) and/or of any bulk material, which may actually be responsible for the effect.
Furthermore, we will subsequently denote any type of relative heat capacity with a small c and the total heat capacity (in the context of buildings sometimes referred to a “thermal mass”) of an object with capital C [J/K].
As is well know, we distinguish between molar heat capacity cn in [J/(mol·K)], i.e., heat capacity relative to the number of atoms or molecules, specific heat capacity mcp in [J/(kg·K)], i.e., heat capacity relative to mass, and volumetric heat capacity vcp in [J/(m3·K)] i.e., heat capacity relative to volume. Obviously, the volumetric heat capacity of a substance can be derived from its specific heat capacity by multiplying it with its density ρ in [kg/m3]. These are isobaric heat capacity values, as indicated by the index p.
It is interesting to note that for most simple (mono-atomic) solids the molar heat capacity is approximately constant, as expressed by Dulongs-Petit's law
                                             c            n                    ≈                    ⁢                      3            ·            R                                                                    ≈                        ⁢                                          3                ·                8.3                            ⁢                                                          ⁢                              J                /                                  (                                      mol                    ·                    K                                    )                                                              =                      25            ⁢                                                  ⁢                          J              /                              (                                  mol                  ·                  K                                )                                                        with R being the universal gas constant. There are of course significant differences with respect to specific and volumetric heat capacity.
A few practical values of material, which are relevant for buildings, shall serve as example:
TABLE 1Density, specific and volumetric heat capacityof some materials and substances (typical values,given at T = 300 K and p = 100 kPa)mcpvcpρ[kJ/[kWh/[kJ/[kWh/[kg/m3](kg · K)](kg · K)](m3 · K)](m3 · K)]wood (pine,5002.10.58 10−310500.29dry)concrete2,2000.720.20 10−316000.44brick2,0000.90.25 10−318000.50steel (SS310)7,8000.480.13 10−337001.0 air (dry)1.21.00.28 10−31.20.000, 33water1,0004.2 1.2 10−342001.16
All given solid materials, specifically wood, steel and concrete, are being produced in a wide variety of specific forms, compositions, and qualities, and exhibit thus considerably different mechanical and thermal properties. The given numbers shall only serve to illustrate practically possible values and are rounded to 2 significant digits.
We give these values with energy expressed in kJ (=kWs) as well as kWh, which is very helpful to put it conceptually and practically in relation to the effort and expense related to heating and cooling.
The combined influence of thermal conductivity k in [W/(m·K)] and volumetric heat capacity ρ·cp in [J/(m3·K)] is referred to as thermal inertia, or sometimes as thermal effusivity, and defined as e=(k·ρ·cp)0.5.
Predominantly enclosed spaces, such as conventional buildings, with immobile surface elements with constant surface and/or volumetric properties, i.e. surfaces and structural elements which do not dynamically change their orientation, reflective properties, or other caloric or thermal properties, result in a thermal behavior of the enclosed space, which is to a large part determined by (a) the (constant) spectral reflective properties of its surfaces, and (b) its (constant) total heat capacity, effectively integrating the absorbed portion of the incident electromagnetic power. In particular, the amplitude of temperature changes inside predominantly enclosed spaces, and their delay with respect changes of the irradiance, largely depends on the heat capacity of the structure (and thermal conductivity of the walls).
In general, a large total heat capacity of a predominantly enclosed space is desirable to reduce temperature variation on the inside. For example, a medieval church or halls in castles with massive stone walls, often with a thickness on the order of 1 m or even more, and thus very high heat capacity (“thermal mass”) will have relatively modest temperature swings on the inside, which follow slowly (integrated over many days) the averaged changes in solar irradiance and ambient air temperature on the outside. Such buildings tend to have relatively low air temperatures on the inside, even during summer and/or at tropical and subtropical locations.
The large ratio of heat capacity of the structure itself compared to the heat capacity of the contained air, helps to stabilize the temperature of the air, i.e. the building can e.g. cool or heat inflowing air for a relatively long time (primarily via radiation from and direct contact with its walls).
We will illustrate this point with a very crude first order approximation by assuming said church or hall to be a cube with 25 m outside edge length and 1 m wall thickness (which protrude into said cube volume), made from bricks, and assuming the material parameters values given in Table 1. The ratio of the volume of all 6 walls Vwall=(25 m)3−(25 m−2 m)3=3458 m3 to the volume of the enclosed air Vair (25 m−2 m)3=12167 m3 is 1:3.5, yet the ratio of the thermal capacity of the wall to the enclosed air 426:1. Specifically, in this case the wall would have a thermal capacity of Cwall≈1730 kWh/K and the enclosed ≈12,200 m3 air have a thermal capacity of Cair≈4.1 kWh/K.
To exemplify the implication, we assume further that in one particular instance the walls are at Twall=20° C.=293.1 K, and that the enclosed air is suddenly completely exchanged with air at Tair=40° C.=313.1 K, i.e. ΔT=20 K (e.g. outside air in a equatorial region). Abstractly assuming ideal, complete thermal exchange between the gas and the wall (incl. no thermal exchange with the outside, etc.), this would result in a new equilibrium at approximately Twall=Tair=20.05° C., i.e. the temperature change of the walls is only ΔTwall=0.05 K. Correspondingly often such a gas exchange could occur, before the cooling (or heating) effect of the structure diminishes.
In contrast, the temperature changes inside a simple shack, made primarily from thin steel plates, which also relatively high thermal conductance, will under the same outside conditions have relatively high amplitudes and relatively closely follow the change in radiative power input. Such habitats are frequently found in developing countries.
Similarly, many trailers and mobile homes have at least partially relatively thin metal walls.
In the US, many residential homes are made from relatively thin walls from plywood and/or, gypsum drywall/sheet rock, often in conjunction with insufficient insulation, have in fact relatively low thermal capacity, which makes such buildings very susceptible to changes in solar irradiance and ambient temperature changes, and also results in increased energy expenditure for air conditioning and heating. (Material for thermal insulation, such as glass wool etc, is in general not primarily used to contribute to the thermal capacity of such house, instead it aims at reducing thermal conductance, which changes the heat flux per thermal gradient across the wall, and thus only the required time to reach a new equilibrium, if ever.)
We will again illustrate this point with a very crude first order approximation by assuming the house to be a cube with 8 m (outside) edge length and 3.5 cm wall thickness, made from wood (pine plywood), and again assuming the material parameters values given in Table 1. The ratio of the volume of all 6 walls to the volume of the enclosed air is now 1:37, and the ratio of the thermal capacity of the wall to the enclosed air is only 23:1, about 18× lower compared to the previous example.
Specifically, in this case the wall would have a thermal capacity of only Cwall≈3.9 kWh/K and the enclosed 505 m3 of air have a thermal capacity Cair≈0.17 kWh/K. To exemplify the implication, we assume again that in one particular instance the walls are at Twall=20° C.=293.1 K, and that the enclosed air is suddenly completely exchanged with air at Tair=40° C.=313.1 K, i.e. ΔT=20 K. Again abstractly assuming ideal, complete thermal exchange between the gas and the wall (incl. no thermal exchange with the outside, etc.), this would now result in a new equilibrium temperature of approximately Twall=Tair=20.82° C., i.e. the temperature change of the wall ΔTwall=0.82 K, i.e. the temperature change is correspondingly about 18× higher than in the previous example. Correspondingly fewer gas exchanges could occur, before the cooling (or heating) effect of the structure diminishes.
For obvious economic reasons (price of raw material, transport, installation), it is desirable to use as little as possible material (concrete, stone, clay, steel, aluminum, glass, wood, various insulation materials etc. to erect a structure of given size. In most cases this will be determined by requirements on the structural integrity and stability. On the other hand, as pointed out above, this implies in many cases a relatively low heat capacity of the structure. Therefore, additional methods and predominantly passive elements (in terms of expenditure of external energy) are desirable, which in some cases reduce the amount of solar radiation, which is absorbed by a structure. In addition, relatively inexpensive elements and methods to increase the thermal capacity and/or to dynamically redistribute thermal energy are desirable.
Several elementary forms of passive cooling are well known. Historic examples from the Middle-East include so-called “wind chimneys”. More recently passively ventilated roof structures of buildings have been introduced in the United States, which can in some circumstances contribute somewhat to reducing the temperature increase inside a building, in particular in attic space, under given solar radiation levels. However, existing designs, which were essentially empirically derived (in contrast to CFD computer simulations or laser- or sound-based flow measurements), are far from optimal in terms of size, shape, and placement of the air in- and outlets as well as the resulting airflow patterns.
As an additional, principle limitation, solely passive (or driven/“active”) ventilation based methods cannot achieve temperature lower than ambient air temperature.